The present invention relates to the field of natural gas liquefaction. Liquefaction of natural gas consists in condensing the natural gas and in subcooling it to a temperature that is low enough for the gas to remain liquid at the atmospheric pressure. It is then transported in LNG carriers.
The international liquid natural gas (LNG) trade is currently developing rapidly, but the whole of the LNG production chain requires considerable investment. Reduction of this investment is therefore a priority objective.
Document U.S. Pat. No. 6,105,389 provides a liquefaction method comprising two cooling mixtures that circulate in two closed and independent circuits. Each circuit works by means of a compressor supplying the cooling mixture with the necessary power for cooling the natural gas. Each compressor is driven by a gas turbine selected from among the commercially available standard ranges. However, the power of the gas turbines currently available is limited.
The present invention is aimed to improve the method disclosed by document U.S. Pat. No. 6,105,389 in order to increase the liquefaction power while keeping the standard compressors.
One object of the present invention is to allow to reduce the investment cost required for a liquefaction plant. Another object of the present invention is to carry out, under better conditions, separation of the nitrogen that may be contained in the gas.
The principle of the method according to the invention consists in condensing and in subcooling the natural gas under pressure by indirect heat exchange with one or more cooling mixtures. However, subcooling is performed to a temperature such that the natural gas does not remain entirely liquid after expansion to the atmospheric pressure. In the method according to the invention, the liquefied natural gas under pressure is expanded in at least two stages so as to obtain at least two gas fractions. At least one gas fraction is recompressed and mixed with the natural gas prior to condensation.
The present invention provides a natural gas liquefaction method comprising the following stages:
a) combining the natural gas with a compressed gas obtained in stage f) to obtain a mixture of natural gas,
b) cooling the natural gas mixture and a second cooling mixture by indirect heat exchange with at least a first cooling mixture so as to obtain a cooled natural gas and a cooled second cooling mixture, then
condensing and cooling the cooled natural gas by indirect heat exchange with the cooled second cooling mixture and with a first gas fraction obtained in stage c) so as to obtain a liquefied natural gas under pressure,
c) expanding the liquefied natural gas under pressure obtained in stage b) to obtain a liquid fraction and the first gas fraction,
d) cooling the liquid fraction obtained in stage c) by indirect heat exchange with a second gas fraction obtained in stage e) so as to obtain a cooled liquid fraction and a heated second gas fraction,
e) expanding the cooled liquid fraction obtained in stage d) to obtain a liquefied natural gas and the second gas fraction,
f) compressing at least part of the heated second gas fraction obtained in stage d) to obtain the compressed gas.
The liquefied natural gas under pressure obtained in stage b) can be at a temperature that is higher by at least 10xc2x0 C. than the bubble-point temperature of the liquefied natural gas obtained in stage e) at the atmospheric pressure.
The liquefied natural gas under pressure obtained in stage b) can be at a temperature ranging between xe2x88x92105xc2x0 C. and xe2x88x92145xc2x0 C., and at a pressure ranging between 4 MPa and 7 MPa.
In stage f), part of the first gas fraction obtained in stage c) and part of the heated second gas fraction obtained in stage d) can be compressed to obtain a compressed gas.
A denitrogenation treatment can be applied to the liquid fraction and to the first gas fraction obtained in stage c) to enrich the first gas fraction with nitrogen.
In stage b), the natural gas mixture can be condensed and cooled by indirect heat exchange with the first cooling mixture and a second cooling mixture, the second cooling mixture being condensed by indirect heat exchange with the first cooling mixture. In stage d), the liquid fraction obtained in stage c) can be cooled by heat exchange with the second gas fraction obtained in stage e) and with the second cooling mixture.
In stage a), the natural gas can be at a temperature ranging between 30xc2x0 C. and 60xc2x0 C., and at a pressure ranging between 4 MPa and 7 MPa.
The natural gas mixture and the second cooling mixture can be cooled to a temperature ranging between xe2x88x9235xc2x0 C. and xe2x88x9270xc2x0 C. by heat exchange with the first cooling mixture.
In stage c), said liquefied natural gas under pressure can be expanded to a pressure ranging between 0.2 MPa and 1 MPa and, in stage e), said liquid fraction can be expanded to a pressure ranging between 0.05 MPa and 0.5 MPa.
The first cooling mixture can comprise the following components in molar fraction:
The second cooling mixture can comprise the following components in molar fraction:
In fact, the method according to the invention allows to significantly increase the production capacity by adding a limited number of additional equipments.
The method according to the invention is particularly advantageous when each cooling circuit uses a cooling mixture that is entirely condensed, expanded and vaporized.